[SCOTI]

On the C20/30's, the factory leaf pads are boxed in. For C10's, they're not.

Where the flip bracket typically 'tucks' under the leaf pad on a C10, the bracket for the C20/30 butts up against the outside edge of the pad & gets welded @ the point of contact on the ends (this info should be in the instructions).

If it were me, to keep it simple I'd put the brackets in a vice & slightly bend both ends until the bracket seats against the axle tube @ the bottom. The notched portion does not fit perfectly as they're made for the largest axles tubes (I believe the dana70 axle tubes are larger OD vs the 14bolt). Once the ends of the bracket are welded to the OE leaf pad & the center of the tube is seated against the center of the flip bracket, you could weld a small bead where your thumb is in the above pic for further assurances.

The more involved alternative, would be to disect the bracket & re-make it so each end fits, re-make the c-notch plates to the exact size you want, & then weld it back together.